The present invention relates to a liquid crystal display having at least two retardation film layers.
Liquid crystal materials are useful for electronic displays because the light traveling through a layer of liquid crystal (LC) material is affected by the anisotropic or birefringent value (xcex94N) of the material, which in turn can be controlled by the application of a voltage across the liquid crystal material. Liquid crystal displays are desirable because the transmission or reflection of light from an external source, including ambient light and backlighting schemes, can be controlled with much less power than is typically required for illuminating displays. Liquid crystal displays (LCDs) are now commonly used in such applications as digital watches, calculators, portable computers, desktop computers, avionic cockpit displays, car navigation systems, and many other types of electronic devices which utilize the liquid crystal display advantages of long-life and operation with low voltage/power consumption.
The information in many liquid crystal displays is presented in the form of a matrix array of rows and columns of numerals or characters which are generated by a number of segmented electrodes arranged in such a matrix pattern. The segments are connected by individual leads to driving electronics which apply a voltage to the appropriate combination of segments in order to display the desired data and information by controlling the light transmitted through the liquid crystal material.
Graphic information in, for example, avionic cockpit applications or television displays may be achieved by a matrix array of pixels which are connected by an X-Y sequential addressing scheme between two conventional sets of perpendicular conductor lines (i.e. row and column lines). More advanced addressing schemes typically use arrays of thin film transistors, diodes, MIMS, etc. which act as switches to control the drive voltage at the individual pixels.
Contrast ratio is one of the most important attributes determining the quality of both normally white (NW) and normally black (NB) liquid crystal displays. The contrast ratio in a NW display is determined in low ambient conditions by dividing the xe2x80x9coff statexe2x80x9d light transmission (high intensity white light) by the xe2x80x9con statexe2x80x9d or darkened intensity. For example, if the xe2x80x9coff statexe2x80x9d transmission is 200 fL at a particular viewing angle and the xe2x80x9con statexe2x80x9d transmission is 5 fL at the same viewing angle, then the display""s contrast ratio at that particular viewing angle is 40 or 40:1 for the particular driving voltage utilized.
Accordingly, in normally white (NW) LCDs, the primary factor adversely limiting the contrast ratio is the amount of light which leaks through the display in the darkened or xe2x80x9con statexe2x80x9d. In normally black (NB) liquid displays, the primary factor limiting the contrast is the amount of light which leaks through the display in the darkened or xe2x80x9coff statexe2x80x9d. The higher and more uniform the contrast ratio of a display over a wide range of viewing angles, the better the LCD.
The contrast ratio problems are compounded in bright environments such as sunlight and other high intensity ambient conditions where there is a considerable amount of reflected and scattered ambient light adjacent the display. The lesser the amount of ambient light reflected from the display panel, the better the viewing characteristics of the display. Therefore, it is desirable to have a LCD reflect as little ambient light as possible. The amount of ambient light reflected by a display panel is typically measured via conventional specular and diffused reflection tests. In color liquid crystal displays, light leakage causes severe color shifts for both saturated and gray scale colors. These limitations are particularly important for avionic applications, where the copilot""s viewing of the pilot""s displays is important.
The legibility of the image generated by both normally black (NB) and normally white (NW) liquid crystal display devices depends on viewing angle, especially in matrix addressed devices with a large number of scanning electrodes. Absent a retardation film, the contrast ratio of a typically NW (and sometimes NB) liquid crystal display is usually at a maximum only within a narrow viewing or observing envelope centered about normal (0 degrees horizontal viewing angle, 0 degrees vertical viewing angle) and drops off as the angle of view increases.
It would be a significant improvement to provide a liquid crystal display capable of presenting a uniform high quality, high contrast ratio image over a wide field of view with little or no ambient light reflection.
Normally black (NB) twisted nematic displays typically have better contrast ratio contour curves or characteristics than do their counterpart NW displays in that the NB displayed image can be better seen at larger viewing angles. However, NB displays are much harder to manufacture than NW displays due to their high dependence on the cell gap xe2x80x9cdxe2x80x9d of the liquid crystal material, as well as on the temperature of the liquid crystal material itself. Accordingly, a long felt need in the art has been the ability to construct a NW display with high contrast ratios over a large range of viewing angles, rather than having to resort to the more difficult to manufacture NB display to achieve these characteristics.
What is generally needed in normally white displays is an optical compensating or retarding element(s), i.e. retardation film, which introduces a phase delay that restores the original polarization state of the light, thus allowing the light to be blocked by the output polarizer in the on state. Optical compensating elements or retarders are known in the art. It is known that the polyimides and copolyimides can be used as negative birefringent retarding elements in normally white liquid crystal displays and are said to be custom tailorable to the desired negative birefringent values without the use of stretching. The polyimide retardation films may be uniaxial but with an optical axis oriented in the Z direction, i.e. perpendicular to the plane defined by the film.
The type and orientation of optical compensation or retardation normally used depends in part upon the type of display, normally black or normally white.
In a normally black (NB) twisted nematic display, the twisted nematic liquid crystal material is placed between polarizers whose transmission axes are parallel to one another. In the un-energized OFF state (no voltage above the threshold voltage Vth is applied across the liquid crystal material), normally incident light from the backlight is first polarized by the rear polarizer and in passing through the pixel or cell has its polarization direction rotated by the twist angle of the liquid crystal material dictated by the buffing zones. This effect is known as the twisting effect. The twist angle is set, for example, to be about 90 degrees so that the light is blocked or absorbed by the front or output polarizer when the pixel is in the OFF state. When a voltage is applied via electrodes across the normally black pixel, the liquid crystal molecules are forced to more nearly align with the electric field, eliminating the twisted nematic optical effect of the LC material. In this orientation, the optical molecular axes of the liquid crystal layer molecules are perpendicular to the cell walls. The liquid crystal layer then appears isotropic to normally incident light, eliminating the twist effect such that the light polarization state is unchanged by propagation through the liquid crystal layer so that light can pass through the output polarizer. Patterns can be written in a normally black display by selectively applying a variable voltage to the portions of the display which are to appear illuminated.
Turning again to normally white (NW) LCD cells, in a normally white liquid crystal display configuration, a twisted nematic cell preferably having a twist angle of about 80 degrees to 100 degrees (most preferably about 90 degrees), is placed between polarizers which have substantially crossed or perpendicular transmission axes, such that the transmission axis of each polarizer is either parallel (P-buffed) or perpendicular (X-buffed) to the buffing direction or orientation of the liquid crystal molecules in the interface region of the liquid crystal material adjacent each polarizer. In other words, normally white cells can be either P-buffed where both polarizer axes are substantially parallel to their respective adjacent buffing zones, or X-buffed where both polarizer axes are substantially perpendicular to their respective adjacent buffing zones.
This NW orientation of the polarizers reverses the sense of light and dark from that of the normally black displays previously discussed. The OFF or un-energized (no applied voltage above Vth across the liquid crystal material) areas appear light in a normally white display, while those which are energized appear dark.
The problem of ostensibly dark areas appearing light or colored when viewed at large angles still occurs, however, thereby creating the aforesaid lowered contrast ratios at reasonably large viewing angles. The reason for the reduced contrast ratios at large viewing angles in normally white displays is different than the reason for the problem in normally black displays. In the normally white energized darkened areas, the liquid crystal molecules tend to align with the applied electric field. If this alignment were perfect, all of the liquid crystal molecules in the cell would have their long axes normal to the glass substrate or cell wall. In the energized state, the normal white display appears isotropic to normally incident light, which is blocked by the crossed polarizers, thus, resulting in a darkened pixel or sub-pixel.
The loss of contrast with increased viewing angles in normally white pixels or displays occurs primarily because the homeotropic liquid crystal layer does not appear isotropic to OFF axis or OFF normal light. Light directed at OFF normal angles through the liquid crystal material propagates in two modes due to the anisotropy or birefringence (xcex94N) of the liquid crystal layer, with a phase delay between these modes which increases with the incident angle of light. This phase dependence on the incident angle introduces an ellipticity to the polarization state which is then incompletely extinguished by the front or exit polarizer in the normally white cell, giving rise to light leakage. Because of the normally white symmetry the birefringence has no significant azimuthal dependence.
FIG. 1 is a contrast ratio curve graph of a prior art normally white twisted nematic light valve. The light value for which the contrast ratio curves are illustrated in FIG. 1 included a rear linear polarizer having a transmission axis defining a first direction, a front or light-exit linear polarizer having a transmission axis defining a second direction wherein the first and second directions were substantially perpendicular to one another, a liquid crystal material having a call gap xe2x80x9cdxe2x80x9d of about 4.20 um, a rear buffing zone (i.e. orientation film) oriented in the second direction, and a front buffing zone oriented in the first direction. The LC material was Model No. MLC-6256 available from Merck. The temperature was about 25.0 degrees C for the graph illustrated in FIG. 1. The light valve did not include a retarder. The above lighted parameters with respect to FIG. 1 are also applicable to FIGS. 2 and 3.
The contrast ratio of FIG. 1 illustrates a driving voltage of 5.0 volts, i.e. Von, a 0.0 volt xe2x80x9cOFF statexe2x80x9d Voff voltage, and a backlighting with white light. As shown in FIG. 1, at least about 10:1 contrast ratios extend along the 0 degree vertical viewing axis only to angle of about xe2x88x9240 degrees horizontal and about +40 degrees horizontal. Likewise, at least about 30:1 contrast ratios extend along the 0 degree vertical viewing axis only to horizontal angles of about xc2x130 degrees. FIG. 1 illustrates the common problems associated with typical normally white liquid crystal displays in that their contrast ratios at large horizontal and vertical viewing angles are limited.
FIG. 2 is a contrast ratio curve plot of the same normally white light valve described above with respect to FIG. 1. However, the FIG. 2 plot was formulated utilizing a Von of about 4.5 volts and a Voff of about 0.0 volts. Again, the temperature was about 25.0 degrees C and white light was used. As can be observed by comparing the graphs of FIGS. 1 and 2, as the xe2x80x9con statexe2x80x9d voltage applied to the liquid crystal material decreased, as in FIG. 2, the contrast ratio curves expanded horizontally and contracted vertically. If the voltage is decreased to about 4.0 volts or less both the horizontal and vertical contrast ratios contract.
The 10:1 contrast ratio area of FIG. 2 along the 0 degree vertical viewing axis extends a total of about 85 degrees (from about xe2x88x9245 degrees to +40 degrees horizontal) as opposed to only about 80 degrees in FIG. 1. Also, the 30:1 contrast ratio area of FIG. 2 along the 0 degree vertical viewing axis extended horizontally about 70 degrees as opposed to only about 60 degrees in FIG. 1, the 30:1 ratio being, of course, represented by the contour lines. With respect to vertical viewing angles, the contrast ratio areas of 10:1 and 30:1 in FIG. 2 did not extend along the 0 degree horizontal viewing axis to the negative vertical extent that they did in FIG. 1. In sum, the normally white light valve of FIGS. 1 and 2, with the parameters specified, had less than desirable contrast ratios at large viewing angles, these contrast ratios expanding horizontally and contracting vertically as the xe2x80x9cON statexe2x80x9d or driving voltage across the liquid crystal material decreased.
FIG. 3A is a grey level intensity (fL) versus vertical viewing angle plot of the prior art light valve described above with respect to FIGS. 1-2, this plot illustrating the gray level behavior of the prior art light valve. The various curves represent horizontal viewing angles from about xe2x88x9260 degrees to +60 degrees along the 0 degree vertical viewing axis. FIG. 3B illustrates the same data as FIG. 3A for vertical viewing angles using a driving voltage versus intensity (fL) plot.
Gray level performance and the corresponding amount of inversion are important in determining the quality of a LCD. Conventional liquid crystal displays typically utilize anywhere from about eight to 256 different driving voltages. These different driving voltages are generally referred to as xe2x80x9cgray levelxe2x80x9d voltages. The intensity of light transmitted through the pixel or display depends upon the driving voltage. Accordingly, gray level voltages are used to generate dissimilar shades of color so as to create different colors when, for example, these shades are mixed with one another.
Preferably, the higher the driving voltage in a NW display, the lower the intensity (fL) of light transmitted therethrough. Likewise then, the lower the driving voltage, the higher the intensity of light reaching the viewer. The opposite is true in normally black displays. Thus, by utilizing multiple gray level driving voltages, one can manipulate either a NW or NB liquid crystal display to emit desired intensities and shades of light. A gray level Von is generally known as any driving voltage greater than Vth (threshold voltage) up to about 4.0-6.5 volts.
Gray level intensity in LCDs is dependent upon the displays"" driving voltage. It is desirable in NW displays to have an intensity versus driving voltage curve wherein the intensity of light emitted from the display or pixel continually and monotomically decreases as the driving voltage increases. In other words, it is desirable to have gray level performance in a NW pixel such that the intensity (fL) at 6.0 volts is less than that at 3.0 volts, which is in turn less than that at 2.0 volts, etc. Such good gray level curves across wide ranges of viewing angles allow the intensity of light reaching the viewer via the pixel or display to be easily and consistently controlled.
Referring again to FIG. 3A, the grey level intensity versus viewing angle plot illustrated therein of the prior art light valve of FIGS. 1-2 having no retardation film(s) are undesirable because of the curves cross each other. At the same viewing angles, the crossing results in increases in the intensity as the voltage increases within particular voltage ranges, which are known as inversions.
A theoretically perfect driving voltage versus intensity curve with respect to a NW display would have decreased intensity (fL) for each increase in gray level driving voltage at all viewing angles. In contrast, FIG. 3 illustrates the inversion in intensity of radiation emitted from the light valve for each corresponding increase in gray level driving voltage in a range of voltages dependent upon viewing angle. Accordingly, it would satisfy a long felt need in the art, if such a liquid crystal display could be provided with little or no inversion.
FIG. 4 is a schematic illustration showing an optic arrangement of a normally white liquid crystal display device disclosed in U.S. Pat. No. 5,570,214. As illustrated, the LCD includes a rear polarizer 111, a rear retardation plate or film 113, a liquid crystal cell 119 including a liquid crystal material sandwiched between a rear orientation or buffing zone oriented in direction A0 and a front orientation or buffing zone oriented in direction A1, a front retardation film 114, and a front polarizer 112.
The rear polarizer 111 is provided at the light incident side of the liquid crystal layer 119, a front or exit polarizer 112 is provided at the light exit side of the liquid crystal layer 119, a rear retardation film 113 is provided between the liquid crystal layer and the polarizer 111, and a front retardation film 114 is provided between the liquid crystal layer and the front polarizer 112. This prior art NW display may be referred to as xe2x80x9cP-buffedxe2x80x9d or parallel buffed because the rear polarizer transmission axis P1 is parallel to the rear orientation direction A0, and the front polarizer transmission axis P2 is parallel to the front orientation direction A1.
The product of parameters xe2x80x9cxcex94Nxe2x88x92dxe2x80x9d of the liquid crystal layer 119 is set in the range of 450-550 nm. The liquid crystal material of U.S. Pat. No. 5,570,214 is left handed as defined in the art. The aligning direction of the rear orientation film on the light incident side of the liquid crystal layer 119 is a rubbing direction Ao inclined at approximately 45xc2x0 with respect to the horizontal x-direction of the liquid crystal cell. The aligning direction of the orientation or buffing film on the front side of the liquid crystal layer is oriented in direction A1 which is rotated about 90xc2x0 in a counterclockwise direction from the orientation direction A0 of the orientation film on the rear side of the liquid crystal material. Therefore, the liquid crystal layer 119 sandwiched between the opposing orientation films is twisted substantially 90xc2x0. The pretilting angle of the liquid crystal molecules is approximately 1xc2x0.
The rear linear polarizer 111 has a transmission axis P1 which is parallel to the orientation direction A0, while the front polarizer 112 has a transmission axis direction P2 which is parallel to the front orientation direction A1. The transmission axes of the front and rear polarizers 112 and 111 are perpendicular to one another thereby defining a normally white liquid crystal display. The rear retardation plate or film 113 is so arranged that its optical axis R1 is either parallel to or crosses at 90xc2x0 to the rear rubbing direction A0. The front retardation film 114 is so arranged that its optical axis R2 is either parallel to or crosses at 90xc2x0 to the rubbing direction A1. These retardation films 113 and 114 are formed to have equal retardation values (dxe2x88x92xcex94N) where xe2x80x9cdxe2x80x9d is the thickness of the retardation film and xe2x80x9cxcex94Nxe2x80x9d is the anisotropic or birefringent value of the retardation film. The retardation values of the retardation films 113 and 114 are set in the range of 300-400 nm. The front and rear retardation films are formed of the same material such as, for example, a polycarbonate or polyvinyl alcohol, and the outer surfaces thereof are preferably covered with a protective film made of triacetyl cellulose or the like.
The orientation or buffing directions of prior art FIG. 4 are xe2x80x9csix o""clock buffed.xe2x80x9d The term xe2x80x9csix o""clock buffedxe2x80x9d means that the rear and front orientation directions A0 and A1 are oriented in directions so as to provide a viewing zone having an extended region in the six o""clock area of the graphs shown in FIGS. 5A-5D. In other words, because the orientation direction A0 goes from the upper left to the lower right as shown in FIG. 4, and orientation direction A1 goes from lower left to upper right, the resulting viewing zone has better contrast as shown in FIGS. 5A-5D in the negative vertical region below the 0xc2x0 vertical viewing axis. This is what is meant by the phrase xe2x80x9csix o""clock buffed.xe2x80x9d
Alternatively, if the orientation direction A0 went from the lower right to the upper left, and the orientation direction A1 was directed from the upper right to the lower left, then the display of FIG. 4 would have been xe2x80x9ctwelve o""clock buffedxe2x80x9d and would have provided a viewing zone having better contrast ratios in the positive vertical viewing angles instead of the negative vertical viewing angles. The six o""clock buffed LCDs of FIGS. 4 and 5A-5D illustrate viewing zones with better contrast ratios in the negative vertical area below the 0xc2x0 vertical viewing axis as opposed to the positive vertical viewing area above the 0xc2x0 vertical viewing axis.
In the prior art liquid crystal display of FIG. 4, the contrast ratios are measured in FIGS. 5A-5D for the four possible cases of retardation film orientation, when the value of dxe2x88x92xcex94N of a liquid crystal layer 119 is set to 510 nm and the retardation value of both retardation films 113 and 114 is set to 350 nm (the value measured by the light having a wavelength of 589 nm). The four cases are as follows.
FIG. 5A shows contrast ratio curves for the case where the optical axes of the rear and front retardation films 113 and 114 are disposed together in parallel to the rear rubbing direction A0. The solid or outer contrast ratio curve in FIGS. 5A-5D represents a contrast ratio of 10:1. The inner or equally broken contrast curve in FIGS. 5A-5D represents a contrast ratio of 100:1. The intermediate contrast ratio in FIGS. 5A-5D represents a contrast ratio of 50:1. Furthermore, in the graphs of FIGS. 5A-5D, each circle represents a 10xc2x0 shift in viewing angle. In other words, the center of the graph represents a 0xc2x0 vertical and 0xc2x0 horizontal viewing angle, the first circle represents 10xc2x0, the second circle 20xc2x0, etc. As can be seen in FIG. 5A, the 10:1 contrast ratio curve extends horizontally along the vertical 0xc2x0 viewing axis to about xe2x88x9237xc2x0 and +40xc2x0, and extends upwardly along the 0xc2x0 horizontal viewing axis to about 15xc2x0 vertical.
FIG. 5B shows contrast ratio curves for the case where the optical axis R1 of the rear retardation film 113 is disposed in parallel to the orientation direction A0, and the optical axis R2 of the front retardation film 114 is disposed perpendicular to the rubbing direction A0. The direction R1 is parallel to the rear polarizer axis P1, and R2 is parallel to the front polarizer axis P2. As can be seen in FIG. 5B, the 10:1 contrast ratio curve extends along the 0xc2x0 horizontal viewing axis only to about 15xc2x0 vertical. Also, the 50:1 contrast ratio curve extends along the 0xc2x0 horizontal viewing axis only to about 5xc2x0 vertical.
FIG. 5C shows contrast ratio curves for the case where the optical axes of the rear and front retardation films 113 and 114 are arranged in parallel with one another and cross at 90xc2x0 to the rear buffing direction A0. In FIG. 5C, the 10:1 contrast ratio curve extends upward along the 0xc2x0 horizontal viewing axis only to about 15xc2x0 vertical. Also, the 10:1 contrast ratio curve extends along the 0xc2x0 vertical viewing axis a total of about 75xc2x0-80xc2x0.
FIG. 5D shows contrast ratio curves for the case where the optical axis R1 of the rear retardation film 113 is arranged to cross at 90xc2x0 to the rubbing direction A0, and the optical axis R2 of the front retardation film 114 is arranged in parallel to rear orientation direction A0. In FIG. 5D, the 10:1 contrast ratio curve extends horizontally along the 0xc2x0 vertical viewing axis a total of about 60xc2x0-65xc2x0. Also, the 10:1 contrast ratio curve in FIG. 5D extends upward along the 0xc2x0 horizontal viewing axis only to about +15xc2x0vertical.
It is important to note that FIGS. 4 and 5 only teach techniques suitable for retardation films that are uniaxial, and do not suggest any applicability to non-uniform films, as discussed below. Uniaxial films have 180 degree symmetry in the plane and therefore are illustrated by a double arrow in the figures. However, non-uniform films are not suitable to be reversed by 180 degrees while necessarily providing the same optical properties. Moreover, the direction of a non-uniform film in relation to the respective buffing direction is not necessarily simply reversible, as in uniaxial films. Non-uniform films have significantly different optical properties than uniaxial films.
The rotation of uniaxial retardation films adjusts the viewing zones of LCDs. For example, U.S. Pat. No. 5,184,236 teaches rotating the optical axes of retardation films xc2x115xc2x0 or less when two such films are disposed on a single side of the liquid crystal material. The axes of the uniaxial retardation films are rotated either in the clockwise or counterclockwise direction for the purpose of adjusting the viewing zone. However, when the uniaxial retardation films of the ""236 patent are rotated, the symmetry of the viewing zone is substantially distorted thereby creating viewing zones which are not substantially symmetrical about the 0xc2x0 horizontal viewing axis.
FIG. 6 illustrates the angular relationship between the horizontal and vertical viewing axes and angles described herein relative to a liquid crystal display and conventional LCD angles xcfx86 and xcex8. The +X, +Y, and +Z axes shown in FIG. 6 are also defined in other figures herein. Furthermore, the xe2x80x9chorizontal viewing anglesxe2x80x9d (or XANG) and xe2x80x9cvertical viewing anglesxe2x80x9d (or YANG) illustrated and described herein may be transformed to conventional LCD angles: azimuthal angle xcfx86, and polar tilt angle xcex8, by the following equations:
TAN(XANG)=COS(xcfx86)*TAN(xcex8)
SIN(YANG)=SIN(xcex8)*SIN(xcfx86)
or
COS(xcex8)=COS(YANG)*COS(XANG)
TAN(xcfx86)=TAN(YANG)/SIN(XANG)
It is noted that XANG and YANG may be referred to as horizontal (H) and vertical (V) angles in the east pole coordinates.
Referring to FIG. 7, Sharp Corporation has developed a liquid crystal configuration, generally referred to as the O-mode, using wide view retardation films (described in detail later) made by Fuji-Film, Japan. The wide view retardation films from Fuji-Film, Japan include discotic layers with variable orientation of tilt angle of the uni-axial optical vector. The optical axis R1 of the rear retardation film 150 is disposed in a parallel orientation to the orientation direction A0, and the optical axis R2 of the front retardation film 152 is disposed in an opposite direction to the rubbing direction A1. The direction of R1 is perpendicular to the rear polarizer axis P1, and R2 is perpendicular to the front polarizer axis P2. It is noted that P1 and P2 are the transmission axis the O-mode liquid crystal display was primarily developed for laptop display panels and has reasonably good vertical and horizontal contrast ratios, operating in a binary operation, for its intended market. For example, a typical display includes a horizontally xc2x145 degrees and vertically (+30 degrees, xe2x88x9240 degrees) having a contrast ratio of about 30:1. One example of an O-mode performance is shown in FIGS. 8-10. In particular, FIG. 8 illustrates the display angular intensity distribution when driven in the black mode, FIG. 9 illustrates the display angular intensity distribution when driven in the white mode, while FIG. 10 illustrates the resulting contrast ratio as a function of viewing angles.
While such a contrast ratio range is acceptable for laptop displays, the avionic industry generally regards such liquid crystal devices as non-acceptable. The grey level separation near black for O-mode liquid crystal displays at above normal viewing angles is poor. This is undesirable for applications where accurate grey levels are important for the full range of operation, such as avionics where the pilot observes the display from a raised position, such as the second seat. The reason for the poor grey level performance, at least in part, is that the compensation provided by the retardation films for the molecule alignments of the liquid crystal material is designed for one particular driving voltage. When driving at different voltage levels the particular retardation films do not compensate adequately for the changed characteristics of the liquid crystal material. In addition when viewing at angles below normal, the contrast ratio of the O-mode display decreases significantly. Moreover, when viewing the display at extreme angles below normal there is a tendency for inversion, namely, the white becomes black, and black becomes white. Significant inversion characteristics are simply unacceptable for mission critical application, such as avionics. Also, the O-mode display has a tendency to change xe2x80x9cwhitesxe2x80x9d to xe2x80x9cyellowxe2x80x9d because the cell gap is normally optimized for the green wavelength to provide approximately 90 degrees of polarization rotation, with the cell gap normally providing less than 90 degrees of rotation for red wavelengths and providing greater than 90 degrees of rotation for the blue wavelengths. Accordingly, the polarizer and retardation films do not adequately compensate for the different wavelengths, especially when the colors include grey-levels mainly at wide horizontal angles.
FIG. 11 is an exploded schematic view of the optical components, and their respective azimuthal xcfx86 orientations of an existing twisted nematic (TN) NW LCD, this LCD being either a light valve (LV) or an active matrix liquid crystal display (AMLCD) having a matrix array of pixels and colored (e.g. RGB, RGBG, RGGB, or RGBW) sub-pixels therein. As shown, this display includes from the rear forward toward viewer 180, conventional backlight 181 rear or light-entrance linear polarizer 182, rear tilted negative retarder 183, rear negative retarder 184 (which may be either uniaxial or biaxial, including indices of refraction nx, ny, and nz), rear tilt sense or direction AR which shows the tilt sense of the liquid crystal molecules adjacent the rear LC orientation layer, twisted nematic (TN) liquid crystal (LC) layer 185, front tilt sense or direction AF which shows the tilt sense of the LC molecules of layer 185 adjacent the front LC orientation film, front negative retarder 186 which may be uniaxial or biaxial, front tilted negative retarder 187, and a front or light-exit linear polarizer (analyzer) 188. The LCD or LV of FIG. 11 is viewed by viewer 180. Such a device is disclosed in U.S. Pat. No. 5,990,997, incorporated by reference herein. However, the size and shape of the contrast region disclosed by U.S. Pat. No. 5,990,997 is not ideal for many applications. Accordingly, a desire still exists for devices with different configurations that provide optimized viewing angles and grey level separation for particular applications.
What is desired, therefore, is a liquid crystal device having wider horizontal viewing angles with increased vertical viewing angles. In addition, the device should provide better gray level separation with no inversion in the upper hemisphere. Also, the device should have significantly reduced yellow effect at wide horizontal viewing angles. Further, the device should have increased left to right symmetry of the contrast ratio contours with little yellow effect at wide viewing angles, such as xc2x160 degrees.
The present invention overcomes, at least in part, the aforementioned drawbacks of the prior art by providing, in one embodiment, normally white twisted nematic liquid crystal display (LCD) that includes front and rear orientation layers sandwiching the liquid crystal layer therebetween. The front orientation layer causing a front liquid crystal tilt sense direction AF and the rear orientation layer causing a rear liquid crystal tilt sense direction AR different than the direction AF. The rear and front tilted retardation layers are located on opposite sides of the liquid crystal layer. Each of the rear and front tilted retardation layers has an optical axis defining an azimuthal angle xcfx86, and a tilt angle xcex8, where at least said tilt angle xcex8 varies through the thickness of the layer. The azimuthal angle aspect xcfx86 of an optical axis of the rear tilted retardation layer is oriented substantially anti-parallel relative to the rear tilt sense direction AR of liquid crystal molecules proximate said rear orientation layer, and the azimuthal angle aspect xcfx86 of an optical axis of the front tilted retardation layer is oriented substantially parallel relative to the front tilt sense direction AF of liquid crystal molecules proximate the front orientation layer. Each of the rear and front tilted retardation layers includes a tilt angle xcex8 which is substantially greater on the side of the tilted retardation layer closest to the liquid crystal layer than on the side furthest from the liquid crystal layer. The rear and front polarizing elements, each of which has an optical axis, located on opposite sides of the liquid crystal layer, and the optical axes of the rear and front polarizing elements being substantially perpendicular to one another.
The present invention overcomes the aforementioned drawbacks of the prior by providing, in one embodiment of the present invention, a normally white twisted nematic liquid crystal display (LCD) that includes a liquid crystal layer for twisting at least one normally incident wavelength of visible light when in the off-state. Front and rear orientation layers sandwich the liquid crystal layer therebetween with the front orientation layer causing a front liquid crystal tilt sense direction AF and the rear orientation layer causing a rear liquid crystal tilt sense direction AR different than the direction AF. Rear and front tilted retardation layers are located on opposite sides of said liquid crystal layer. Each of the rear and front tilted retardation layers has an optical axis defining an azimuthal angle xcfx86, and a tilt angle xcex8, where at least the tilt angle xcex8 varies through the thickness of the layer. The azimuthal angle aspect xcfx86 of an optical axis of the rear tilted retardation layer is oriented substantially anti-parallel relative to the rear tilt sense direction AR of liquid crystal molecules proximate the rear orientation layer, and the azimuthal angle aspect xcfx86 of an optical axis of the front tilted retardation layer is oriented substantially parallel relative to the front tilt sense direction AF of liquid crystal molecules proximate the front orientation layer. At least one of the rear and front tilted retardation layers includes a tilt angle xcex8 which is substantially greater on the side of the tilted retardation layer closest to the liquid crystal layer than on the side farthest from the liquid crystal layer.
In another embodiment of the present invention, at least one of the azimuthal angle aspect xcfx86 of an optical axis of the rear tilted retardation layer is oriented substantially perpendicular relative to the rear tilt sense direction AR of liquid crystal molecules proximate the rear orientation layer, and at least one of the azimuthal angle aspect xcfx86 of an optical axis of the front tilted retardation layer is oriented substantially perpendicular relative to the front tilt sense direction AF of liquid crystal molecules proximate the front orientation layer.
In another aspect of the present invention, the front and rear orientation layers sandwich the liquid crystal layer therebetween, with the front orientation layer causing a front liquid crystal tilt sense direction AF and the rear orientation layer causing a rear liquid crystal tilt sense direction AR different than the direction AF. The first and second tilted retardation layers are located on the same side of the liquid crystal layer. Each of said first and second tilted retardation layers has an optical axis defining an azimuthal angle xcfx86 and a tilt angle xcex8, where at least the tilt angle xcex8 varies through the thickness of the layer. The azimuthal angle aspect xcfx86 of an optical axis of the first tilted retardation layer is oriented substantially anti-parallel relative to the rear tilt sense direction AR of liquid crystal molecules proximate the rear orientation layer.